1. Field of the Invention
This invention relates to a flotation process for de-inking printed waste paper by treating the paper pulp in the pulper at alkaline pH values by means of alkali-metal silicate, oxidative bleaches, alkali metal salt solutions and/or amine salt solutions of protein fatty acid condensates or dispersions of alkaline earth metal salts of protein fatty acid condensates as well as separating the loosened printing ink particles from the fibrous material suspension by flotation.
2. Description of the Related Art
De-inking refers to the selective removal of printing ink from the waste paper fiber suspension. The reusable material is processed into the finished products, e.g. writing paper, printing paper, toilet paper, etc depending on the individual starting material. The essential steps of the de-inking process are at first, the removal of the printing inks from the fibers in a pulper by means of suitable chemicals and then selectively separating the printing inks from the fiber suspension. Both in the flotation- and the wash-de-inking processes, these steps are carried out in an alkaline medium, where the printing ink is removed in flotation cells or washers. The reusable material obtained is then processed into the desired products.
A commonly used chemical formulation can be as follows: (quantities referred to the dry matter content of the waste-paper)
0.5 to 2.0% by weight of alkali-metal hydroxide (100%) PA0 0.2 to 1.5% by weight of soap or fatty acid (100%) PA0 0.05 to 0.1% by weight of nonionic dispersing agent (100%) PA0 2.0 to 5.0% by weight of alkali-metal silicate (water glass) PA0 0.1 to 0.3% by weight of diethylenetriaminepentaacetic acid Na salt (DTPA) (40%) PA0 0.5 to 3.0% by weight of oxidizing bleaching agent (100%)
The effects of the individual chemicals in the flotation process are as follows:
alkali metal hydroxide, preferably sodium hydroxide, causes the fibers to swell thus making it easier to loosen the inter-fiber bonds as well as to separate the printing-ink. This is further promoted by the neutralization or saponification of printing ink binders. Furthermore, sodium hydroxide provides the necessary alkalinity for the neutralization of the fatty acids, which is necessary for their function as collectors. Nonionic dispersing agents cause improved wetting of the fibers by lowering the surface tension of the water, which promotes inter alia the action of the other chemicals, and which essentially results in the loosening of the printing inks and their dispersion. By the dispersion of the printing inks re-absorption into the fibers is prevented or makes it difficult for them to become reattached to the fibers.
The separated pigments are hydrophobized by the collectors, i.e. usually fatty acids or soaps, which makes selective removal possible. With the anionic collectors previously used, together with the alkaline medium, the water hardness formers are of decisive importance for this function, because only their calcium salts cause hydrophobing of the printing ink particles, thus making the latter floatable. Oxidizing bleaching agents prevent or compensate for the yellowing of the wood-containing constituent of the waste paper and in addition--depending on the amount used--cause further bleaching of the fibers. Alkali silicates and DTPA (Diethylene-triaminepenta-acetic acid) are used for their stabilization, which prevent a too rapid decomposition of the bleaching agents by complexing the heavy metals.
According to the end product desired, newspapers, magazines, computer paper, files, etc., but in most cases mixtures of these are used as waste-paper, both the printing-ink system employed and their age playing a decisive role in the possibility of de-inking them.
After the de-inking process the paper is evaluated by measurement of the degree of whiteness, in which the whiteness of the test sheets is compared to that of a standard (e.g. barium sulfate with measurement of the degree of whiteness according to Elrepho) and given in percentages. For example, a waste paper mixture of newspapers and magazines in the ratio 1:1 gives a whiteness of 56 to 60%, where the whiteness of the unprinted margins of this waste paper amounts to approximately 65 to 68%.
German patent application No. DE-OS 29 03 150 a process is described for the de-inking of printed waste paper by treating the paper pulp in the pulper at alkaline pH values by alkali silicate, oxidizing bleaching agents, higher fatty acids or their salts and nonionic dispersing agents, and separation of the loosened printing ink particles from the dyestuff-suspension by flotation, where a fatty acid alkanol amide is additionally used in the pulper.
On the other hand U.S. Pat. No. 4,586,982 describes a similar process, which is differentiated from the above named DE-OS 29 03 150 in that no fatty acid alkanol amide is used in the pulper and in that the fatty acids and/or resin acids and the dispersing agents are used in the form of an oil-in-water-emulsion. The known processes, however, show several disadvantages.
The use of solid fatty acids as collectors necessitates melting apparatus or a heated storage container if they are supplied already melted. Heating is not necessary if fatty acids are used, with a melting point below the normal temperature range, these, however, usually present disadvantages for industrial use. The liquid fatty acids which have a higher melting point only saponify slowly under the reaction conditions in the pulper, such that the soaps forming on the surface of the fatty acid particles make the further saponification of the enclosed fatty acids more difficult and can even prevent it. This fact does not only result in a reduction of the collector effect, but can also lead to an increased flotation of the fibers and felt pollution, through the hydrophobing of the fibers by the unsaponified fatty acids.
In order to overcome these difficulties, according to the prior art saponification units are connected after the storage tanks. The resulting aqueous soap solution must then be permanently heated, until processes, because the latter forms gels at normal temperature, i.e. the intermediate storage containers as well as the dosing pipes including the dosing-head of the dosing pump must be thermally insulated and continuously heated.
If solid soaps are used, a saponification plant is no longer necessary. As the complete solubility of the soap granules is not given in most cases in the conditions dependent on the process, dissolving plants must be installed to prevent negative effects. The disadvantages described above still apply to the resulting soap solution. A further considerable disadvantage of the known methods is to be seen in that the above mentioned formation of calcium soap of the fatty acid, which were first effective as collectors, only successively takes place during the de-inking process in the pulper with the hardness-former of the water used. The reaction time required for this increases the total length of time necessary for the process; an incomplete reaction causes a worse de-inking results, which are also reflected in lower degrees of whiteness. Processes, in which the fatty acids are used in the form of an emulsion also have these disadvantages.
Furthermore in the German Patent Application D 11549 IVa/55b a process for the regeneration of waste paper by an alkaline aqueous liquor is described. The liquors to be used here contain: (a) water glass, (b) at least one oxygen-evolving compound, such as hydrogen peroxide, sodium peroxide, perborate, per-carbonate, (c) at least one protective colloid containing nitrogen in an amide or sulfonamide type bond, (d) at least one nitrogen-free organic compound with a wetting- and washing-effect. A possible protective colloid (component c) is a condensation product of a soluble protein substance or of a higher molecular protein cleavage product with a soap-forming fatty-, resin- or naphthene acid or with an aliphatic or aliphatic-aromatic sulfonic acid. In particular alkyl benzenesulfonic acid salts with 10 to 18 C-atoms in the alkyl radical are used as nitrogen-free wetting agents and detergents. The compounds used in this process as protective colloids reduce the fiber damage or fiber loss which is otherwise observed.
The process described in this German Patent application refers to so-called "wash-de-inking", in which the printing-ink is dispersed in the best possible way by pulping of the waste paper. The printing ink remains in the liquor and is then washed out with the liquor in the subsequent draining stage.
As opposed to this, the process according to the present invention is concerned with what is known as "flotation-de-inking", in which the different wetabilities of the printing-ink and the paper fibers is used for their separation. The hydrophobic printing-ink is flocculated and collected and then separated in the flotation foam.
Details of the differences of the two de-inking processes can be found in: "Ullmanns Encyklopadie der technische Chemie", 4th Edition, Volume 17 (1979), pp. 570 to 572, as well as in "Tappi" (Technical Association of the Pulp and Paper Industry"), Volume 63 (1980), No. 9, pp. 113 to 116. According to this the real difference between the wash process and the flotation process consists in the printing inks separation principle. The actual de-inking chemical in the wash process is a detersive dispersing agent, which is responsible for producing a stable dispersion of separated printing-ink particles and for preventing a renewed adsorption of these particles to the fibers. By contrast the actual de-inking chemical in the flotation process is the so-called collector, which is responsible for causing a specific adsorption of the printing-ink particles in the air bubbles. The two processes are therefore not comparable with each other either as such or with respect to the mode of action of the chemicals used.
With respect to the above mentioned prior art it is an object of the present invention to make available an improved flotation process for de-inking printed waste paper, which does not have the above mentioned disadvantages and furthermore leads to outstanding whiteness levels. In particular the object of the present invention is also to make possible the use of protein fatty acid condensates as collectors, which do not necessitate melting plants or heated storage containers. The present problem can be solved in that in the flotation process for de-inking printed waste paper, protein fatty acid condensates in the form of their alkali metal salts and/or amine salts, if necessary combined with dispersing agents, in molecular distribution in an aqueous solution liquid at normal temperature or in the form of their alkaline earth metal salts which are finely distributed, if necessary together with dispersing agents, in a dispersion which is fluid at normal temperature are used.